1. Field of the Invention
The present invention relates to touch screens, and more particularly, and to a touch screen having low base mutual-capacitance and a related touch sensing control circuit for performing touch sensing operations.
2. Description of the Prior Art
A capacitive touch screen consists of an insulator such as glass, coated with a transparent conductor such as indium tin oxide. As the human body is also an electrical conductor, touching the surface of a touch screen results in a distortion of the electric field of the touch screen, measurable as a change in capacitance. Different technologies may be used to determine the location of the touch.
Projected capacitive touch technology is one of most popular technologies, which allows more accurate and flexible operation in touch sensing. An X-Y grid is formed either by etching one layer to form a grid pattern of electrodes, or by etching two separate, perpendicular layers of conductive material with parallel lines to form the grid.
Typically, there are two types of projected capacitive touch sensing technologies: self-capacitance and mutual-capacitance. In a mutual-capacitance touch sensing system, driving lines and sensing lines are used to measure mutual-capacitance. The driving lines are usually disposed in a direction perpendicular to another direction in which the sensing lines are disposed, thereby forming a grid pattern having a plurality of intersections, where the driving lines and the sensing lines are separated by a non-conducting layer. During touch sensing operation, a driving signal is applied to the driving lines on a line-by-line basis. Accordingly, charges impressed on the driving line due to the driving signal capacitively couple to the sensing lines that are intersected. This leads to a measurable current and/or voltage on the sensing lines. The relationship between the driving signal and signal measured on sensing lines is related to the capacitance coupling the driving and sensing lines. It is therefore possible to measure the capacitances (i.e., mutual-capacitance) of coupling capacitors at the intersections according to the driving signal and the measured sensing signals.
When bringing a touch object (e.g. a finger or conductive stylus) near a surface of the touch sensing system, it changes a local electric field around certain intersections, which reduces the capacitances measured at these intersections. A touch location can be therefore accurately determined according to capacitance changes. Generally, a capacitance change is found by monitoring a variation of a base capacitance at each intersection, where the base capacitance is the capacitance measured at an intersection when there is no touch object close to the intersection. In the touch sensing system, an analog-to-digital converter (ADC) is typically used for quantizing the capacitance, and the ADC usually needs certain resolution to quantize the variation of the base capacitance to a certain degree of precision.
At present, the touch sensing systems that are used widely includes an on-cell touch sensing system which is attached outside of a display device and an in-cell touch sensing system which is embedded in the display device. The in-cell touch sensing system more compactly integrates the touch sensing system with the display device, and therefore the driving lines are very close to the sensing lines. This allows the base capacitance to become larger than the base capacitance of the on-cell touch sensing system. When the capacitance change is relatively tiny compared to the base capacitance, the touch sensing system requires a high resolution ADC to quantize the variation of the base capacitance. For example, assuming that the base capacitance at an intersection is 21.4 pF, the capacitance change due to a touch is 0.1 pF and the capacitance change needs to be quantized to 256 levels, it is necessary to use an ADC having at least 16-bit resolution to convert the capacitance change of 0.1 pF into 256 levels (256/2^16=0.1/21.4). As the manufacturing cost of the ADC depends on the resolution of the ADC, this undesirably increases the manufacturing cost of touch sensing system.